Earthquake Engineering: Can A Building Withstand 1994 Northridge Earthquake?

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BUFFALO, N.Y. — In a cavernous, warehouse-type building here at
the University of Buffalo, a group of men and women put on hard
hats and waited for a 6.7-magnitude earthquake to strike. But
this was no ordinary quake, generated by the rupture of faults
deep in the Earth — these rumblings were being created on
purpose.

Researchers from Johns Hopkins University spent the summer at the
University of Buffalo's earthquake engineering research center to
see how a two-story building made of
cold-formed steel — thin, lightweight sheets of steel that
are rolled or pressed into beams to construct, among other
things, modern skyscrapers — can withstand
powerful seismic forces.

Over the past several months, Benjamin Schafer and Kara Peterman,
both of the Whiting School of Engineering at Johns Hopkins in
Baltimore, conducted a series of tests on two similarly
constructed buildings — one consisting of just a bare steel
skeleton, and a second with exterior sheathing and nonstructural
elements added, including interior walls, a stairway and a coat
of yellow weather-proofing material.

By mid-August, the researchers had already completed 131
different shake tests on their buildings, but on Aug. 16, Schafer
and Peterman prepared for their final and strongest test yet: a
simulation of the 1994 Northridge earthquake that rocked Los
Angeles, killing 60 people and causing roughly $13 billion in
damage. [ Image
Gallery: This Millennium's Destructive Earthquakes ]

In the earthquake testing facility, the two-story building is
imposing, measuring 50 feet (15 meters) long, 20 feet (6 m) wide,
and 20 feet (6 m) tall. The structure's appearance is
unremarkable — easy to mistake for the first floors of any
budding new development on a construction site — save for a
cluster of heavy-duty chains stemming from the unfinished roof to
an overhead crane that was used to lower the building onto the
lab's massive shake tables. These moving platforms, powered by
hydraulics pumps that are just barely visible through a gap in
the floor, are capable of
replicating the seismic forces of an earthquake.

Known unknowns

In their previous tests, the cold-formed steel structures were
designed to stand up to the smaller quakes that Schafer and
Peterman were creating, but the researchers were now curious how
their second building, outfitted with interior walls and a
stairway, would fare if they recreated the forces such that the
building was sitting almost on top of the
Northridge earthquake's fault line.

"In our other tests, we knew the building was engineered to still
be standing after, so that definitely instills a sense of
confidence, but today is a big question mark," Peterman, a
friendly and talkative civil engineering doctoral student, told
LiveScience with a nervous laugh on the morning of the final
shake test. "We really don't know what's going to happen. We're
hoping it's not catastrophic, at least."

When buildings are designed, structural engineers incorporate
so-called shear walls, which are composed of braced panels and
are used to anchor the building against forces such as strong
winds or seismic activity. But during actual earthquakes
— particularly very intense ones — buildings can act somewhat
erratically, Schafer said.

"No one tells the building which part should resist the
earthquake and which should not," Schafer, the tall and bookish
chair of the department of civil engineering at Johns Hopkins,
and the study's lead researcher, animatedly told LiveScience. "A
lot of the building's parts can get engaged in an earthquake, and
you can't avoid that. For a really big earthquake, most of the
theories say that all the little stuff might get damaged, but
you'll be left with the shear walls that you designed in the
beginning to support it. But, you can't test out that idea until
you have a
big earthquake. Today, we're making a big earthquake."

Shake it

Ten cameras and more than 100 sensors were attached to the
building to record the amount of movement and
damage the structure sustained during the test. When the
platforms began shaking, the two-story building rocked from side
to side, amid loud rumblings and several deafening cracks.

At the end, Schafer and Peterman waited for the sensors to
indicate that the building was still structurally sound before
they stepped inside to inspect the amount of damage. After
carefully examining the walls up close, shining flashlights into
dark corners and bending down to see the joints between walls,
the researchers huddled together to discuss their early
observations before Schafer declared the test a resounding
success.

"There are cracks and a lot of damage in corners, but it's all
pretty cosmetic," Schafer said excitedly after the inspection,
smiling widely, as if surprised by the building's resilience. "In
a few places on the exterior, we can see some damage, and once we
take off the [weather-proofing material], we may be able to see
the shear walls damaged, but the performance was far better than
we would have ever imagined."

Schafer and Peterman will have to tear down their test building
this month to make room for a new team of researchers who will be
moving into the earthquake facility. After enduring a summer of
shake tests, culminating in the simulated Northridge quake, their
building, with its bare walls and lone staircase, has done its
job.

Peterman said she will be sad to see the building torn down, but
she doesn't get attached to her test structures anymore. Still,
the first object on which she ever performed earthquake tests — a
small, palm-size object — sits proudly on her desk at work, she
said.

Earthquake postmortem

Meanwhile, it will take Schafer and Peterman months to pore
through all the data from their sensors, but during their initial
examination of the building, the researchers did encounter
something unexpected on the second floor.

When the structure was designed, 11 concrete blocks, each
weighing 2,000 lbs. (907 kilograms), were piled onto the second
floor to represent furniture and people who might occupy in a
building in a real earthquake, and were in the actual Northridge
earthquake. [ The
10 Biggest Earthquakes in History ]

After the simulated Northridge quake, Schafer and Peterman were
surprised to find that the 2,000-lb. blocks moved about 10 inches
(25 centimeters) from their original positions.

"When the building moved, the blocks were their own thing,"
Schafer explained. "They all moved, including one that donked
into the back wall, which might have been the crack we heard."

In their autopsy of the building, Schafer and Peterman will
scrutinize every piece of the building, including looking for
hidden damage inside the walls. Their results could help
improve nationwide building codes for cold-formed steel
buildings, which are increasingly popular for low- and midrise
buildings. The research could reduce the likelihood of future
catastrophic building collapses in earthquake-prone areas of the
country.

"The end goal is to improve the cold-formed steel seismic design
code," Peterman said. "In the future, we'll be able to more
efficiently design cold-formed steel buildings, because we have
an idea of what's going on. If you have a better picture of how
these buildings react to seismic loads, you'll be able to make
more informed designs."